Billions of pounds of thermoplastic polymers are used annually in the world for producing fibers, films, shaped articles, bottles, etc. Disposal of these polymeric materials by incineration or by placing them in landfills is becoming unacceptable due largely to the ever-increasing environmental impact. Recycling offers many advantages from an environmental standpoint; however, efforts so far have been hampered by the lack of fast, convenient, and economically attractive methods for identification and separation (sorting) of the various thermoplastic polymers currently available in the marketplace, and hence ending up in landfills. It has been estimated that plastics comprise 7.3 percent by weight of all municipal solid waste, of which only about 1 percent is currently recycled (U.S. Congress, Office of Technology Assessment "Facing America's Trash: What Next for Municipal Solid Waste", OTA-0-424 (Washington, D.C.; U.S. Government Printing Office, October, 1989)). Visual identification and manual separation techniques presently used for sorting and separation of plastics are labor intensive and expensive in addition to being subject to human error. Efforts to use some inherent physical property of the plastics such as density for identification and separation have thus far not proven to be attractive. To allow plastics to become truly recyclable, plastic compositions which have unique properties which allow them to be separated from each other by automated methods, as opposed to manual separation, are thus very desirable.
It is known (U.S. Pat. No. 4,540,595) that one may mark documents such as bank checks by the use of inks that fluoresce in the near infrared region, i.e., generally between 650 and 800 nm, for automatic identification. Fluorescent phenoxazine dyes of the formula ##STR1## wherein R.sub.1 and R are alkyl and X.sup.- is an anion are shown to be effective near-infrared fluorescing compounds for this purpose. Attempts to use these compounds for adding to thermoplastics to impart fluorescence in the near infrared have failed because the high temperatures necessary for the preparation and processing of thermoplastics cause decomposition of the phenoxazine compounds.
A method for separation of magnesium bearing ore particles based on the presence of a fluorescent compound, i.e., 8-hydroxyquinoline, is known (U.S. Pat. No. 4,423,814). The conditioned ore is irradiated with UV light to induce fluorescence and the magnesium-rich mineral separated from the lean ore particles by detecting the difference of the fluorescence intensity. This method of identification and sorting, based on the use of compounds which fluoresce in the UV light range is not appropriate for thermoplastics in general because many absorb UV light themselves as well as the absorbance by residual products packaged therein. U.S. Pat. No. 4,321,133 discloses a similar process for sorting limestone ores.
A method for sorting agricultural materials based on irradiation with near-infrared light has also been disclosed (U.S. Pat. No. 4,915,827). Absorption in the infrared region is measured and compared to predetermined infrared absorption criteria, which criteria distinguish the desired material from undesired material. This method does not relate to plastic materials and does not involve near infrared fluorescence as a distinguishing part of the method.
Near infrared absorbing compounds such as carbon black (U.S. Pat. Nos. 4,408,004; 4,476,272; 4,535,118) and iron oxide (Fe.sub.2 O.sub.3) (U.S. Pat. Nos. 4,250,078; 4,420,581) have been added to thermoplastic polyesters in small quantities to improve heat-up rates during molding operations. These near infrared light absorbing compounds are not fluorescent and thus cannot be used as near infrared fluorescing "tags".
Further, it has been proposed that one may separate polyvinyl chloride bottles from polyester bottles based on an x-ray method (R&D Magazine, July 1990, p. 102). The x-ray detector is sensitive to the chlorine found in polyvinyl chloride plastics, but not sensitive to the polyester plastics since they contain no chlorine. Obviously, this separation method is very limited in applicability and involves the use of hazardous x-ray radiation.
Near infrared fluorescing compounds have also been used in immunoassay procedures for identifying cancerous tissue in the human body (U.S. Pat. No. 4,541,438).
Finally, it has been proposed (G. Patonay, Analytical Chemistry, Vol. 63, No. 6, 1991, pp 321-327) to use near infrared fluorescent compounds for fluorogenic labels for biomolecules; however, the disclosed fluorescent compounds have poor thermal stability and are not suitable for tagging or marking thermoplastic compositions.